Method for age-hardening of a superalloy

ABSTRACT

The invention provides a process for age-hardening of a superalloy comprising the steps of treating the superalloy in solution followed by one-dimensional quenching in oil. The superalloy then undergoes rapid induction aging in a controlled Argon atmosphere medium-frequency induction furnace.

FIELD OF THE INVENTION

[0001] The present invention relates to an improved method for age-hardening of a superalloy and more particularly, an improved method for age-hardening of a Nickel-base superalloy by using rapid induction heating.

BACKGROUND OF THE INVENTION

[0002] Superalloys which can be iron-base, cobalt-base, or nickel-base alloys have several advantageous properties, compared with alloys, which include high-temperature properties, oxidation resistance and creep resistance.

[0003] Increasingly, superalloys are required to be capable of operating in severe environments. Some of the properties that are required for operating in such environments include high temperature strength, creep resistance, fatigue resistance, excellent ductility, good impact resistance and resistance to hot corrosion. An example of an application of a superalloy in such an environment is the use of nickel-base superalloys in gas turbine blades and vanes. In such an environment the superalloys must be capable of operating at temperatures above 1,000° C., with short-term peaks above 1,100° C.

[0004] One particular class of superalloys is the Nickel-base superalloys. Cast nickel-base superalloys are usually composed of high volume fractions of γ′ (Ni₃(Al,Ti)) phase coherently precipitated in a Face-Centered Cubic (FCC) matrix, together with eutectic phases and one or more carbide phases. The desired properties and resistance to microstructural changes at high temperatures in these alloys are obtained through contribution of all phases particularly γ′ precipitates with suitable morphology, structure, shape, size and distribution. Optimum characteristics and morphology of secondary phases in superalloys will be obtained by applying suitable age-hardening treatments.

[0005] Some of the rapid heating processes that are used for the age-hardenable materials include resistance rapid heating, laser heating, and salt-bath heating. Such processes are disclosed, for example, in H. Matzner and M. H. Braunau “Tests on the heat treatment of extrusions by jouleage”, Aluminium, 73, (1997) 214-216; V. V. Medvedev, Yu. B. Sazonav and S. A. Konstantinova, “Structural changes during short-term super heating of high temperature Nickel-base alloys”, Steel in the USSR, Vol. 15 (1985) 442-443; and in A. G. Rakhshtadt, O. M. Zhigalina, O. M. Khovova and G. M. Klykov, “Effect of quenching after rapid electrocontact heating on the microstructure of age-hardenable 36NkhTYu alloy with a mixed mechanism of precipitation of the strengthening phase”, The Physics of Metals and Metallography, Vol. 81, No. 5 (1996) 557-563. All references referred to in the description are hereby incorporated by reference.

[0006] A standard age-hardening process of a cast superalloy, for example the Nickel-base superalloy IN738LC, includes solution treatment of the superalloy at about 1125° C. for 2 hours followed by three dimensional air-cooling to room temperature. The superalloy is then age-hardened in a furnace, with a gradual heating of approximately 400° C./hr, at about 850° C. for 24 hours.

[0007] This process is a lengthy process and can be costly and time consuming during the manufacturing of a superalloy.

SUMMARY OF THE INVENTION

[0008] A process is provided that provides not only time and energy savings during the age-hardening process but also a better performance and mechanical properties due to a better microstructure, in comparison with the standard process that is used.

[0009] The present process for age-hardening a superalloy includes the steps of treating the superalloy in solution followed by one-dimensional quenching of the superalloy in oil and rapid induction aging of the superalloy.

DETAILED DESCRIPTION OF THE INVENTION

[0010] In the present invention, a new method has been developed for the age-hardening process of a superalloy, in particular a nickel-base superalloy, for example the nickel-base superalloy IN7638LC, however any superalloy is suitable as will be understood by those of skill in the art. The process includes treating the superalloy in solution followed by one-dimensional quenching in oil. The superalloy then undergoes rapid induction aging in a controlled Argon atmosphere using a medium-frequency induction furnace.

[0011] In a preferred embodiment of the present invention the process includes treating a cast Nickel-base superalloy in a solution treatment at 1125° C. for 2 hours followed by one-dimensional quenching in oil to room temperature. It will be understood by those of skill in the art that the time and temperature of the solution treatment will depend on the type of alloy used. It will also be understood that lower times and temperatures will cause incomplete solution of secondary phases and therefore deficient super saturated solid solution after quenching. It will be further understood that greater times and temperatures will lead to lower quality of mechanical properties due to over growth of grains during solution and/or local oxidation or melting of grain boundaries and phases.

[0012] In the preferred embodiment, in order to achieve the one-dimentional quenching, four sides of the cubic specimens are covered with ceramic material and two parallel sides are subsequently quenched in oil. One-dimentional quenching omits the hydrostatic stresses of the quenching liquid applied to the specimen and increases vacancies density in a preferred direction. In the aging step it causes the increase in the rate of diffusion and also in the nucleation and growth rate of precipitates in that preferred direction.

[0013] In the preferred embodiment, the superalloy then undergoes rapid induction aging in a controlled Argon atmosphere medium-frequency induction furnace in the range of 750° C. to 800° C. for 2 to 15 minutes, at a heating rate of approximately 30° C./Sec. The heating rate, temperature and time of the rapid induction aging of the specimens in the induction furnace are programmed and controlled by an electronic programmable devive, which is well known in the art, that is connected to the induction furnace and the thermocouples. It will be understood by those of skill in the art that the times and temperature ranges will vary for different alloys.

[0014] The effect of rapid heating together with electromagnetic forces applied to the superalloy due to the induction heating, results in a decrease in time and temperature of age-hardening process and an improvement in microstructure of the superalloy in comparison with the standard process.

[0015] Some possible advantages to the process of the present invention include high rates of nucleation and growth of γ′ precipitates and a higher rate of increasing of hardness, in particular within the initial two minutes. Within two minutes of rapid induction aging primary γ′ precipitates may have improved characteristics including an average diameter of 380.6±40 nm and approximately 204+20 particles per 100 (μm)² field area. The precipitates may have spherical or cubic shape and may be in uniform distribution in a matrix with a high volume fraction of approximately 25.6%. In addition to the primary precipitates, secondary precipitates may be produced with improved characteristics including an average diameter of 14±6 nm with approximately 1220±60 particles per one (μm)² field area. The secondary precipitates may have a spherical shape and may be in uniform distribution in a matrix with a high volume fraction of approximately 18.8%.

[0016] Other potential advantages to the process of the present invention include the improvement in the characteristics and morphology of secondary phases at lower temperatures and within a short time. The process provides a more cost-effective method of age-hardening process of a superalloy than standard processes and consequently improves the production efficiency and rate of the superalloy.

[0017] While the embodiment discussed herein is directed to a particular implementation of the invention, it will be apparent that variations of this embodiment are within the scope of the invention. 

We claim:
 1. A process for age-hardening of a superalloy comprising the following steps: i. treating the superalloy in solution; ii. one-dimensional quenching the superalloy in oil; and iii. rapid induction aging of the superalloy.
 2. A process for age-hardening of a superalloy according to claim 1, wherein the superalloy is a nickel-base superalloy.
 3. A process for age-hardening of a superalloy according to claim 2, wherein the superalloy is treated in solution for 2 hours at a temperature of about 1125° C.
 4. A process for age-hardening of a superalloy according to claim 1 wherein the superalloy is one-dimentionally quenched in oil to room temperature.
 5. A process for age-hardening of a superalloy according to claim 1, wherein the rapid induction aging includes placing the superalloy in a controlled Argon atmosphere medium-frequency induction furnace.
 6. A process for age-hardening of a superalloy according to claim 5, wherein the rapid induction aging superalloy is in a controlled atmosphere at a temperature in the range of 750° C. to 800° C. for between 2 and 15 minutes.
 7. A process for age-hardening of a nickel-base superalloy comprising the following steps: i. treating the nickel-base superalloy in solution for 2 hours at about 1125° C.; ii. one-dimensional quenching the nickel-base superalloy in oil to room temperature; and iii. rapid induction aging of the nickel-base superalloy in a controlled Argon atmosphere medium-frequency induction furnace in the range of 750° C. to 800° C. for between 2 and 15 minutes.
 8. A superalloy manufactured by the process according to claim
 1. 9. A nickel-base superalloy manufactured by the process according to claim
 1. 10. A nickel-base superalloy manufactured by the process according to claim
 7. 